The present invention relates to a system for measuring flows of fluids which comprise more than one discrete phase. In particular, the invention relates to a system for measuring two or three phase flows from hydrocarbon wells.
The problems associated with measuring the flow rate of a multiphase fluid are well known. Techniques which are applicable to single phase flows, such as impeller systems and the like, have been shown to be highly inaccurate in multiphase flows, and often more than one measurement must be made to enable an accurate measurement of the volumetric flow rates of the various phases. In the oil and gas industry the measurement of multiphase flow is routinely encountered and in the case of producing wells it is common for the produced fluid to comprise a mixture of oil, water and often gas as well. Since it is important to determine the volumetric flow rates of the individual phases at times throughout the life of the well to determine if any remedial action is required to improve or restore the productivity of the well and to assess exactly how much oil is being produced, various methods of flow measurement have been proposed
Typically, the fluids are fed to separators and the volumes of each phase determined separately. However, this requires a large installation and is not able to give an instantaneous determination of the current production from a well. A typical surface system fluid from the well head passes through a choke manifold which can either be a separate system or part of a heater-steam exchange. The fluid is then divided into individual phases with a three phase separation and the flow rates measured at separation temperature and pressure. Samples of the fluids at these conditions are taken, the oil sample being used to determine the oil "shrinkage factor". The oil phase is passed to a gauge tank at atmospheric pressure which permits calibration of the flow meters and atmospheric oil samples to be taken.
Systems for measuring multiphase flow are described in U.S. Pat. No. 4,856,344. The system described in this patent comprises a gradio-venturi flow meter arrangement which, by means of differential pressure measurements along its length, provides information concerning the flow rates of the phases. A homogenizer comprising a step discontinuity in the pipe diameter is provided upstream of the measurement section in order to ensure that the separate phases are well mixed in the measurement section.
In GB 1,272,152, GB 1,461,537 and in the Paper 6.2 North Sea Flow Measurement Workshop 1990, National Engineering Laboratory, Glasgow entitled "Simple Full-Bore Water-Cut Measurement Technique" by D. Brown and J. J. der Boer, there are proposed various systems for making measurements on multiphase flows. In each case, the apparatus has the form of an inverted "U", and a series of pressure measurements are made at differing heights in each leg of the "U" and compared to determine some parameter of the flow. Since none of the measurements relate to dynamic pressure changes, determination of the flow rates of the phases is not possible from the pressure measurement alone.
Measurement of the complex impedance of a multiphase fluid can also provide information on the flow rates, but calibration of the meters is important in view of the potentially large effects of relatively small changes in the composition of the fluid phases. In our co-pending European patent application no. 91202327.2 there is described a system in which the flow is directed through a pair of parallel instrumented flow passages. Periodically, one or other of the passages is isolated from the flow and the fluid therein is allowed to separate under the effect of gravity. Measurements are made on the separating mixture at different locations along the passage such that the concentration of each phase and its contribution to the complex impedance can be determined.